The principal aims of this project are to determine the intracellular potential and sodium ion activity of the proximal tubule cell and their relationship to net transepithelial sodium transport. Values are to be compared in the superficial proximal convoluted and straight tubular segments of the rabbit kidney perfused in vitro. Measurements will be made using conventional microelectrodes, and using double barreled ion selective microelectrodes. The experimental maneuvers are designed to investigate the role of the luminal and basolateral membranes in the regulation of intracellular sodium activity and net transepithelial sodium transport, and to compare these properties in the two tubular segments. Experimental protocols involve the use of Na-K,ATPase inhibition with ouabain, low potassium concentrations, and cold temperature to characterize basolateral active transport. The role of sodium coupled organic substrate transport and hydrogen ion transport in determining intracellular sodium activity are the primary questions to be addressed concerning luminal membrane function. The long term goals are to elucidate the basic mechanisms by which cellular ion exchanges and renal transport processes maintain homeostatic control of electrolyte and water balance of the body fluids.